multithreading - 官方 OpenCL 2.2 标准是否支持 WaveFront？

Question

众所周知，AMD-OpenCL 支持 WaveFront(2015 年 8 月):http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2013/12/AMD_OpenCL_Programming_Optimization_Guide2.pdf

The AMD Radeon HD 7770 GPU, for example, supports more than 25,000 in-flight work-items and can switch to a new wavefront (containing up to 64 work-items) in a single cycle.

但是为什么在 OpenCL 标准 1.0/2.0/2.2 中没有提到 WaveFront？

没有一个PDF没有一个字 波前 : https://www.khronos.org/registry/OpenCL/specs/

我还发现:

2013年:https://community.amd.com/thread/160658

OpenCL is a open standard. It still does not support this swizzling concept. It does not even support wavefront/warp yet.

2013年:https://stackoverflow.com/a/19874984/1558037

That's why the concept is not on the OpenCL specification itself.

2011年:https://forums.khronos.org/showthread.php/7211-How-can-i-split-my-work-load-in-a-GPU-with-OpenCL

Standard OpenCL doesn't have the notion of a "wavefront"

2011年:https://www.cvg.ethz.ch/teaching/2011spring/gpgpu/GPU-Optimization.pdf

确实官方OpenCL 2.2标准还不支持WaveFront？

结语 :

OpenCL 标准中没有 WaveFront，但 在 OpenCL-2.0 中，有类似于 WaveFronts 的 SIMD 执行模型的子组 .

第 100 页: http://amd-dev.wpengine.netdna-cdn.com/wordpress/media/2013/12/AMD_OpenCL_Programming_User_Guide2.pdf

6.4.2 Workgroup/subgroup-level functions

OpenCL 2.0 introduces a Khronos sub-group extension. Sub-groups are a logical abstraction of the hardware SIMD execution model akin to wavefronts, warps, or vectors and permit programming closer to the hardware in a vendor-independent manner. This extension includes a set of cross-sub-group built-in functions that match the set of the cross-work-group built-in functions specified above.

Answer 1

他们一定采用了一种更动态的方法，称为 sub-group :https://www.khronos.org/registry/OpenCL/specs/opencl-2.2.pdf

Sub-group: Sub-groups are an implementation-dependent grouping of work-items within a
work-group. The size and number of sub-groups is implementation-defined.

和

Work-groups are further divided into sub-groups,
which provide an additional level of control over execution.

和

The mapping of work-items to
sub-groups is implementation-defined and may be queried at runtime. 

所以即使它没有被称为波前，它现在可以在运行时查询并且

In the absence of synchronization functions (e.g. a barrier), work-items within a sub-group may be serialized. In the presence of sub -group functions, work-items within a sub -group may be serialized before any given sub -group function, between dynamically encountered pairs of sub - group functions and between a work-group function and the end of the kernel.

甚至有时可能会丢失锁步方式。

在这些之上，

 sub_group_all() and
sub_group_broadcast() and are described in OpenCL C++ kernel language and IL specifications.
The use of these sub-group functions implies sequenced-before relationships between statements
within the execution of a single work-item in order to satisfy data dependencies.

说存在某种子组内通信。因为现在 opencl 有子内核定义:

Device-side enqueue: A mechanism whereby a kernel-instance is enqueued by a kernel-instance
running on a device without direct involvement by the host program. This produces nested
parallelism; i.e. additional levels of concurrency are nested inside a running kernel-instance.
The kernel-instance executing on a device (the parent kernel) enqueues a kernel-instance (the
child kernel) to a device-side command queue. Child and parent kernels execute asynchronously
though a parent kernel does not complete until all of its child-kernels have completed. 

最终，像

kernel void launcher()
{
    ndrange_t ndrange = ndrange_1D(1);
    enqueue_kernel(get_default_queue(), CLK_ENQUEUE_FLAGS_WAIT_KERNEL, ndrange,
    ^{
    size_t id = get_global_id(0);
    }
    );
}

您应该能够以您需要的任何大小生成您自己的(升级的？)波前，并且它们与父内核同时工作(并且可以与子组内线程通信)但它们不被称为波前，因为它们不是由硬件硬编码 imho .

2.0 api 规范说:

Extreme care should be exercised when writing code that uses
subgroups if the goal is to write portable OpenCL applications.

这提醒了 amd 的 16 宽 simds 和 nvidia 的 32 宽 simds 与一些虚构的 fpga 的 95 宽计算核心。也许是伪波前？